PM7344 S/UNI-MPH
DATA SHEET
PMC-950449
ISSUE 6
MULTI-PHY USER NETWORK INTERFACE
Figure 15
- Cell delineation State Diagram
correct HCS
(bit by bit)
HUNT
Incorrect HCS
(cell by cell)
PRESYNC
ALPHA
consecutive
incorrect HCS's
(cell by cell)
DELTA
SYNC
consecutive
correct HCS's
(cell by cell)
The values of ALPHA and DELTA determine the robustness of the delineation
method. ALPHA determines the robustness against false misalignments due to
bit errors. DELTA determines the robustness against false delineation in the
synchronization process. ALPHA is chosen to be 7 and DELTA is chosen to be 6
as recommended in ITU-T Recommendation I.432.
Loss of cell delineation (LCD) is detected by counting the number of incorrect
cells while in the HUNT state. The counter value is stored in the RXCP LCD
Count Threshold register. The threshold has a default value of 360 which results
in an E1 format detection time of 77 ms, and a T1 format detection time of 100
ms.
The RXCP descrambles the cell payload field using the self synchronizing
descrambler with a polynomial of x43 + 1. The cell header is not descrambled.
Note that cell payload scrambling is optional in the S/UNI-MPH.
The HCS is a CRC-8 calculation over the first 4 octets of the ATM cell header.
The RXCP verifies the received HCS using the accumulation polynomial, x8 + x2
+ x + 1. The coset polynomial x6 + x4 + x2 + 1 is added (modulo 2) to the
received HCS octet before comparison with the calculated result as required by
the ATM Forum UNI specification, and ITU-T Recommendation I.432.
The RXCP can be programmed to drop all cells containing an HCS error or to
filter cells based on the HCS and/or the 4 octet cell header. Filtering according
to a particular HCS and/or 4 octet header pattern is programmable through the
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56
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